Characteristics and significance of the biomarkers in the source rocks from the Permian Zhanjin Formation in the Qiangtang Basin,northern Xizang

The present paper deals with the characteristics of source rocks and biomarkers in the core samples of the source rocks collected from the Permian Zhanjin Formation in the Qiangtang Basin, northern Xizang, with the emphasis on the origin, sedimentary environments and maturity of the source rocks. The findings have disclosed that the source rock samples collected from the Zhanjin Formation are rich in n-alkanes, isoprenes, terpanes and steranes. The n-alkanes are characterized by a high and low single peak, respectively, and a dominant peak of low carbon numbers including nC₁₅, nC₁₆ and nC₁₇. The C₂₇-C₂₈-C₂₉ series display the "V-shaped" distribution in the regular steranes, with C₂₇ as a dominant peak. The terpanes have the relative abundance of tricyclic terpane > pentacyclic triterpane > tetracyclic terpane, and the ratios of tricyclic terpenes/pentacyclic triterpenes range between 1.1 and 1.96. The primary sources of organic matter are interpreted to come from the lower marine aquatic organisms. The Pr/Ph ratios range from 0.53 to 1.16, with an average of 0.85, suggesting aphytane dominance. The distribution patterns of pentacyclic triterpanes and the ratios of Ts/(Tm + Ts) indicate that the source rocks were formed in the reduction environments. The contents of gammaceranes and the ratios of gammaceranes/αβ-C 30 hopanes have proved that the source rocks were formed in the seawater with a normal salinity. The maturity parameters such as C₂₉ααα20S/(20S + 20R) and C₂₉αββ/(αββ + ααα) ratios for the steranes, C₃₁αβ22S/(22S + 22R) ratios for the terpanes, and Ts/(Tm + Ts) ratios all show that the source rocks are still in the mature stage, which is consistent with the Ro values indicated by the selected samples.